DESCRIPTION (Investigator's Abstract): The general goal of the research is to understand how signals that originate in rods and in each of the three types of cone combine in the various pathways that mediate human visual function. Most work in this area has addressed only the signals originating in foveal cones. A more general model requires that we understand how signals from both rods and cones contribute to channels subserving the full range of visual functions. The proposed experiments investigate several aspects of extrafoveal pathways by means of psychophysical experiments using adult human observers. One set of experiments investigates the properties of putative dual scotopic (rod) pathways, with special attention to differences in their involvement with cone signals and in their importance for spatial, temporal and increment sensitivity. The experiments derive from a model that integrates physiological, anatomical, and psychophysical evidence for dual rod pathways. Whether the experiments support or refute the model, they will provide empirical evidence on the general importance of dual rod pathways. A second set of experiments characterizes the separate components of the adaptation processes by which rod signals regulate incremental sensitivity of cone pathways (cone-rod interaction), using probe-on-flash and other psychophysical techniques. The experiments will also separate adaptation mechanisms that operate in local retinal areas from those operating at greater distance by means of lateral pathways. A third set of experiments determines the involvement of rod signals in opponent pathways, including both spectrally- and chromatically-opponent pathways. The experiments aim to resolve presently conflicting evidence, reveal which types of opponent pathways are influenced by rod signals, and investigate the role of rod signals in human color vision. The general strategy of all experiments is to employ variation of psychophysical task and of spectral, spatial and temporal stimulus parameters of select different mechanisms or pathways for study. Psychophysical tasks include measurements of incremental, temporal (flicker), and spatial (grating) sensitivities. Signals from rods and from identifiable cone types can then be used as probes to describe detection and adaptation pathways and the quantitative rules for combination of rod and cone signals.